Modern wireless devices, including cellular telephones and Wi-Fi networking devices, require components for transmitting and receiving data simultaneously. Radio frequency (RF) processors have been designed to perform these functions without the need for larger components with greater power requirements. There are many phenomena that degrade the performance of RF processors.
One such phenomenon that degrades performance of an RF processor deals with interference. Specifically, some RF processors are used as full duplex transceivers, i.e., a device that can transmit and receive signals simultaneously. In such devices, RF processors have an inherent problem with self-interference: transmission interferes with reception in a process called transmission leakage even though there should be no overlap between the transmission band and the reception band. Leakage results not only from imperfect duplexer performance in isolating the transmit signal from the reception signal, but also from parasitic coupling paths between multiple elements of the transmission circuitry and the reception circuitry which permit leakage of the transmission signal into the reception signal. Contemporary market pressures push for smaller and smaller transceivers, thus compounding the problem as transmission circuitry is pressed ever closer to reception circuitry.
Another set of phenomena deals with device degradation. Specifically, an RF processor that uses a quadrature amplitude modulation (QAM) scheme may have many properties that can change after factory testing and/or calibration as a result of age, temperature and/or environment. The changes to these properties alter the overall performance of the device. The receiving performance of an RF processor may severely degrade if the change to any one of these properties exceeds certain bounds. Non-limiting examples of such changeable properties include in-phase/quadrature (I/Q) imbalance, the DC offset of RF and analog circuits and the relation of the DC offset to the second-order input intercept point (IIP2) of the processor, gains of RF and analog circuits and the corner frequencies of filters.
In a QAM receiver, the signal being processed is a complex signal including a real part and an (orthogonal) imaginary part. The real part of the complex signal corresponds to the I channel and the imaginary part corresponds to the Q channel. Ideally, in a QAM scheme, the in-phase (I) channel and quadrature (Q) channel carry orthogonal, i.e., non-interfering, channels of information. Because the I channel and Q channel are mixed with orthogonal signals from the local oscillator, and are typically processed through separate circuitry, the signal within the I channel may experience a phase delay that is different than the phase delay experienced by the signal within the Q channel. This difference in phase delay and/or gain between the I and Q channels, or I/Q imbalance, creates unwanted distortion in the received signal.
Amplifiers, mixers, attenuators, and some passive devices can generate intermodulation distortion. These distortion products are a result of a nonlinear transfer characteristic. A common specification, related to distortion, for amplifiers and mixers is the intercept point. If the input versus output of a device is displayed graphically on a dB versus dB scale, the slope of the linear portion will be 1. If second order distortion products are displayed on the same scale they will have a slope of 2, third order distortion products will have a slope of 3, etc. In most cases, distortion products above third order are not important but these rules are still valid. The IIP2 is the point where the linear extension of the second order distortion intersects the linear extension of the input verses output line. In other words the IIP2 is the theoretical input level at which the second-order distortion products are equal in power to the desired signals.
The overall gain of the processor may be defined as the ratio of the peak-to-peak measurement of the output signal to the peak-to-peak measurement of the input signal. The corner frequency of a filter is the transition frequency range between the band of frequencies that can pass through the filter with little impedance, i.e., the pass-band, and the band of frequencies that are greatly attenuated, i.e., the stop-band. Again, as discussed above, both the overall gain and corner frequency of the filter may change after factory testing and/or calibration as a result of age, temperature and/or environment. Unless these changes are accurately determined, compensation or calibration for such changes cannot be maximized.
When manufacturing RF processors, many devices are fabricated on a large disc of semiconductor material. The devices are created to be as uniform as possible, but differences of only a few molecules can significantly alter performance of a single device. Once fabricated, the devices are typically factory tested to verify conformance to specifications. If a device is functional, but does not quite meet standardized performance, it may be a candidate for calibration wherein a calibration signal is used to adjust targeted properties.
Once factory tested and/or calibrated, RF processors are installed into a communication system, for example a cell phone, and are sold. Therefore, factory level calibration cannot account for variation of the performance of the calibrated device due to environmental conditions or degradation over time.
After the RF processor has left the factory, e.g., has been installed into a phone and delivered to a customer, limited conventional post-fabrication calibration methods are available. These limited conventional post-fabrication calibration methods include external calibration signals (delivered to the RF processor) or internal calibration signals (generated by the RF processor). These calibration signals may be used to adjust such properties as I/Q) imbalance, the IIP2 of the processor, gains of RF and analog circuits and the corner frequencies of filters within the processor.
Most conventional post-fabrication methods for calibrating RF processors use an external calibration signal, e.g., wherein the phone having the RF processor receives an externally transmitted calibration signal. In cases where an externally transmitted signal is used, the signal must comply with standards designed by government organizations, such as the Federal Communications Commission (FCC), and standards agreed upon by industry groups, such as the United States Telecommunications Industry Association (TIA-USA).
A calibration signal generated and interpreted entirely internal to the RF processor would not be affected by those standards because it would not be transmitted out of the device.
One conventional post-fabrication calibration technique uses an internal calibration signal. This conventional technique transmits the internally generated calibration signal through the main data path transmitter of an RF processor to calibrate the IIP2. This technique has limited use in IIP2 calibration and cannot be used in specific operating conditions because the emitted radiation out of the main transmit path will violate FCC requirements.
What is needed is an RF processor capable of eliminating transmission interference in a reception signal, and of detecting and/or calibrating parameters within the RF processor after factory calibration.
What is additionally needed is an RF processor capable of calibrating more than just the IIP2 with an internal calibration signal after factory calibration.